WordPress.org

By default WordPress doesn't overwrite existing files. Instead, it appends a number to the end of the filename in order to make it unique, e.g., filename1.jpg. That isn't always the desired behavior, so this plugin makes it so that any files uploaded will automatically overwrite existing files that have the same name, rather than creating a second file with a unique name.

NOTE: You have to make a small change to one of WordPress' core files for this to work; see the FAQ for details. If you need help check out the support forums.